Pneumatic conveying systems are used to transport large quantities of particulate material through pipes. Examples of particulate material includes powders, granules, pellets, seeds, beans, nuts, pasta, pet foods, snack foods, and similar. In a common pneumatic conveying system, a fluid, usually air, is blown through the pipes. The air enters the pipes through a filtered air inlet. The particulate material enters the pipes through a material inlet. The air is mixed with the particulate material and together, the air and particulate material moves through the pipes. Both the air and the particulate material exit the pipes through an outlet.
Dilute phase conveying occurs when the particulate material is transported at a relatively high velocity (i.e., above the saltation point). Particles are suspended in the air stream to move at approximately the same velocity as the air stream. This results in a relatively low product to air ratio (i.e., less than 15:1). This also results in high particle-to-particle and particle-to-pipe interaction, and can lead to undesirable particle shear and breakdown.
Dense phase conveying occurs when the particulate material is transported at a relatively low velocity (i.e., well below the saltation point). A positive displacement blower is located upstream from the particulate material inlet. The positive displacement blower increases fluid (air) pressure within the pipes downstream of the positive displacement blower. Because the particles are not suspended in the fluid (air), they drop to the bottom of the pipe. The pressure from the positive displacement blower pushes the particles together into lumps or “slugs” within the pipe. The slugs of material are transported through the pipe with a pocket of fluid (air) between each slug. Dense phase conveying usually achieves a higher product to air ratio than dilute phase conveying. The lower velocities also result in lower particle-to-particle and particle-to-pipe interaction.
Dense phase conveying systems may be generally described as either batch or continuous.
In a batch dense phase conveying system, the particulate material is loaded into a special container, sometimes called a pressure pot. The container is sealed off and pressurized before the particulate material is introduced into the conveyance pipes. After the container is pressurized, the particulate material is introduced into the conveyance system and transported through the pipes. When the container is empty, the conveyance system is purged with a blast of high velocity fluid (air) to remove any lingering particulate material from that batch. The container is depressurized and refilled with a new batch of particulate material. In some of the more sophisticated batch dense phase conveying systems, two or more containers tie into the system so that one may be in a state of depressurization such that it can be refilled while simultaneously another container is in a state of pressurization such that material can be transported through the pipes.
In a continuous dense phase conveying system, the vessel containing the particulate material generally is not pressurized. Instead, the particulate material is contained within a hopper and is introduced into the pipes via a multi-vane rotary airlock. Typically, the airlock provides metering, to some extent, of the particulate material so that the rate of entry into the pipes is somewhat steady. The airlock also controls, but does not eliminate, fluid (air) leakage out of the system. The hopper is filled and refilled with particulate material, at ambient pressure. The particulate material is continuously fed into the pipes via the airlock and continuously extruded through the pipes, thus eliminating the downtime associated with the purge, depressurization, refill, and pressurization of the batch systems.
Ideally, the slugs of particulate material are transported in a wave form flow pattern that is continuous, predictable, and repeating. The air mass flow rate is critically important to achieve a continuous, predictable, and repeating slug wave form flow pattern. The air mass flow rate is regulated to achieve a constant ratio of air mass flow rate to material mass flow rate in order to achieve a steady state of conveying the slugs. Properly controlling the air mass flow (not air velocity) yields the desired steady state of conveyance.
A vacuum throttle is used to regulate air mass flow by controlling the density of the fluid (air) entering the pipes upstream of the positive displacement blower. A positioning mechanism moves a bullet-shaped element into and out of an orifice to create and regulate a low pressure, or vacuum, environment between the air inlet and the positive displacement blower. By controlling the distance that the bullet-shaped element is inserted into the orifice, the air density entering the pipes via the blower is controlled. By controlling the air density of the system, the air mass flow rate is controlled.
Current state-of-the-art vacuum throttled continuous dense phase pneumatic conveying systems encounter several drawbacks. First, air mass flow fluctuates due to blower slippage and airlock leakage. Second, material mass flow fluctuates due to feed rates into the system and bulk density variations. These, and other shortcomings, have been addressed by employing a complex series of valves and sensors or vacuum and pressure transducers on the blower inlet and discharge that provide electrical signals to a programmable logic controller (PLC). The PLC calculates how to move the bullet-shaped element with respect to the orifice in order to achieve the desired system vacuum relative to the discharge pressure.
The inventor has identified an unaddressed need to regulate the air mass flow without reliance on the complexities of the pressure and vacuum transducers and PLC. The inventor has discovered a means for controlling the air mass flow, by varying the position of the bullet in the vacuum throttle, using a solely mechanical control mechanism.